1. Field of the Invention
The present disclosure relates to a method of toughening thermoplastic polyurethane. The present disclosure also relates to articles, and particularly to golf balls, having a toughened thermoplastic polyurethane outer surface.
2. Description of Related Art
Golf ball covers are generally divided into two types: thermoplastic covers and thermoset covers. Thermoplastic polymer materials may be reversibly melted, and so may be used in a variety of manufacturing techniques, such as compression molding, that take advantage of this property. On the other hand, thermoset polymer materials are generally formed by mixing two or more components to form a cured polymer material that cannot be re-melted or re-worked. Each type of polymer material present advantages and disadvantages when used to manufacture golf balls.
Thermoplastic materials for golf ball covers can be usually include ionomer resin, highly neutralized acid polymer composition, polyamide resin, polyester resin, polyurethane resin, and mixtures thereof. Among these, ionomer resin and polyurethane resin are popular materials for golf ball covers.
Ionomer resins, such as Surlyn® products (commercially available from E. I. DuPont de Nemours and Company), have conventionally been used for golf ball covers. For example, Dunlop Rubber Company obtained the first patent on the use of Surlyn® for the cover of a golf ball, U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of ionomer resins in the cover composition of a golf ball, for example, U.S. Pat. Nos. 3,819,768, 4,323,247, 4,526,375, 4,884,814, and 4,911,451.
However, ionomer resin covered golf balls suffer from the problem that the cover surface may be scraped off by grooves on a clubface during repeated shots, particularly with irons. In other words, ionomer covers have poor scuff resistance. Also, ionomer covered balls usually have inferior spin and feel properties as compared to balata rubber or polyurethane covered balls. The use of softer ionomer resins for the cover will improve spin and feel to some extent, but will also compromise the resilience of the golf balls because such balls usually have a lower coefficient of restitution (C.O.R.). Furthermore, the scuff resistance of such softer ionomer covers is often still not satisfactory.
Thermoplastic polyurethane elastomers may also be used as the cover material, as described in (for example) U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282. However, the thermoplastic polyurethane elastomers disclosed therein do not satisfy all the requirements of moldability, hitting feel, control, resilience, and scuff resistance upon iron shots.
US Patent Application Publication No. 2011/0081492 (the '492 publication) disclosed a method of curing polyurethane composition for use in constructing golf balls by i) mixing isocyanate and polyol compounds to produce a polyurethane prepolymer; ii) chemically-curing the prepolymer by reacting it with a hydroxyl-terminated curing agent at a stoichiometric ratio of isocyanate groups to hydroxyl groups of at least 1.20:1.00 to form a composition; iii) applying the composition over the core and allowing it to partially-cure; and iv) moisture-curing the composition to form a fully-cured cover layer comprising a polyurethane/urea composition.
On the other hand, thermoset polymer materials such as polyurethane elastomers, polyamide elastomers, polyurea elastomers, diene-containing polymer, cross-linked metallocene catalyzed polyolefin, and silicone, may also be used to manufacture golf balls. Among these materials, thermoset polyurethane elastomers are popular.
Many attempts, such as are described in U.S. Pat. Nos. 3,989,568, 4,123,061, 5,334,673, and 5,885,172, have been made to use thermoset polyurethane elastomers as a substitute for balata rubber and ionomer resins. Thermoset polyurethane elastomers are relatively inexpensive and offer good hitting feel and good scuff resistance. Particularly, thermoset polyurethane elastomers may present improvements in the scuff resistance as compared to softened ionomer resin blends. However, thermoset materials require complex manufacturing processes to introduce the raw material and then effect a curing reaction, which causes the manufacturing process to be less efficient. Also, thermoset materials are difficult or impossible to recycle.
Thermoplastic polyurethane often is chosen to serve as the outer surface of an article because thermoplastic polyurethane has many desirable properties. In particular, use of thermoplastic polyurethane in golf balls as outer covers layers is because the cover hardness and spin rate can be varied to suit the user. Further, thermoplastic polyurethane is easily processed by injection molding or other processes.
However, thermoplastic polyurethane does not have high strain rate shear resistance. Therefore, when a golf ball having an outer surface comprising thermoplastic polyurethane is struck at an oblique angle, particularly with a highly-lofted club such as a wedge, the surface often fails. Failure mode is made evident by corrugations, or ‘hairs,’ sticking up from the golf ball.
Scuff-resistant, durable materials also are important in other technologies. For example, articles such as livestock identity tags and sports balls of many types require a durable, scuff-resistant surface.
Accordingly, for the foregoing reasons, there is a need in the art for a system and method that addresses the shortcomings of the prior art discussed above.